This invention relates to optical solar reflectors (OSRs) for spacecraft, and to methods for mounting to the body of a spacecraft.
An optical solar reflector (OSR) is a second-surface reflector on a transparent substrate, which is attached to the outer surfaces of a spacecraft for (a) reflecting solar energy incident on the spacecraft (reflection), and (b) radiating heat energy produced in the spacecraft (emission). In the past, glass or quartz OSR tiles, about 1 inch by 2 inches square and about 1/100" thick, silvered or aluminized on the second surface, and with a thin-film transparent, electrically conductive coating on the outer surface, were affixed to outer surfaces of a spacecraft by use of an adhesive resin material treated so that, when cured, it tends to retain some flexibility. The tiles were spaced apart about 1/100 inch, to allow for differential expansion and contraction. An electrically conductive material, such as a conductive silicone resin, was applied to the interstices between OSR tiles, to make electrical contact between the conductive outer surface of the tiles and the body of the spacecraft. This mounting and interconnecting procedure is labor-intensive.
Modern spacecraft tend to have higher-power payloads than older spacecraft, and therefore are often required to maintain temperature in the presence of a greater heat load than in the past. A spacecraft can ordinarily only lose energy by radiation. The radiation efficiency of the glass or quartz OSR tiles is high. However, the gaps between the OSR tiles, which contain the adhesive, are inherently good solar absorbers by comparison with the OSR tiles themselves. The presence of the gaps may result in an increase in total panel solar absorption by as much as 20%, as a result of the relatively high solar absorptance (.alpha.) of the gap. Also, materials in the gaps, such as epoxy resin adhesives and/or silicone paints, tend to darken upon exposure to ultraviolet radiation and charged particles, as occurs in orbit. The darkened material is a good solar absorber.
Maintenance of low weight is of utmost importance in a spacecraft, because there is a maximum allowable launch weight, and excess weight directly subtracts from payload fuel. The fuel portion of the payload of a spacecraft, such as a communications spacecraft, is used to maintain the attitude and/or station of the spacecraft. The life of the spacecraft is essentially terminated when the fuel is expended. Considering the high capital cost of a spacecraft and its launch (which at present may be upwards of 50 millions of dollars), its service value (one million dollars per month, or more) and the small amount of fuel required to maintain attitude and station (a few pounds mass (lbm) per month), the incremental cost of additional thermal control weight is very large. Improved thermal control surfaces are desired.